The human PIN1 gene is a functional homolog of the yeast ESS1 gene. Gene knockout experiments show that ESS1 is essential for cell growth and viability. We have replaced the yeast ESS1 gene with a version of human PIN1 whose expression is conditional. When synthesis of PIN1 in these strains is shut off, the cell arrest with a characteristic morphology; cells appear to be stuck in mitosis. Nuclear division proceeds apparently normally, however, but afterwards nuclear staining material (Dapi) imaged at the BMIRR by high resolution fluorescence epi-illumination appears to breakdown into fragments reminiscent of mammalian cell apoptosis. Same-cell fluorescence and video-enhanced differential interference contrast microscopy shows no change in the general morphology of the cell even in the presence of nuclear breakdown products.